1. Field of the Invention
The present invention relates to a method of calculating the position of a mobile device based on:    the known phases of spreading codes of a plurality of visible satellites,    a known accurate or approximate time reference corresponding to the measurement of these code phases, and    reconstituting the pseudodistances from the time reference, an approximate position of the mobile device to be positioned and the code phases.
2. Description of the Prior Art
In the field of mobile telephones, it is increasingly found to be necessary to be able to locate mobile telephone devices.
To this end, it is known in the art to combine, within this type of mobile device, usually comprising a GSM (Global System for Mobile communications) mobile telephone type cellular radio telephone receiver, a GNSS (Global Navigation Satellite System) receiver such as a GPS (Global Positioning System), GLONASS or GALILEO receiver by means of which the mobile device picks up transmissions from satellites to determine its position. In this way, in the event of a traffic accident, for example, or for any other positioning requirement, the mobile device is able to calculate and transmit its position.
The position of a receiver of the above kind may be determined in the following manner: a plurality of satellites transmit continuously a signal that carries a “time of week” (TOW) and is addressed to the receiver. The receiver when synchronized to the clock of the satellites is able to measure the propagation time of the signal and to deduce therefrom a distance between it and a particular satellite. A receiver of this kind is able to determine its position by means of a process of triangulation using three satellites. Each measured propagation time represents the radius of a sphere centered on a particular satellite, the receiver being located on that sphere. With two distance measurements, the position of a receiver is on a circle formed by the intersection of two spheres. A simultaneous third measurement reduces the intersection to two points, one of which is at a great distance and is easy to isolate.
However, the clock of the receiver is affected by a skew ΔT because it is not totally synchronous with the GPS. The atomic clocks of GPS satellites are very accurate but the accuracy of the more rudimentary GPS receiver is much lower. This clock skew ΔT is the time difference between the clock of the receiver and the clock of the satellites, and may be as much as several seconds. It is reflected in an error in the measurement of the GPS signal propagation time and thus an error c.ΔT in the satellite-receiver distances, where c is the velocity of light. This error affects all distances measured by the receiver. Since the distances are imperfect, as they are subject to a time skew, they are referred to as pseudodistances. The time skew, which is unknown a priori, must then be determined.
There is therefore a fourth unknown in three dimensions (because there are three satellites) and it is necessary to measure at least one additional distance, and thus to have access to at least four satellites, in order to solve a system of four equations in four unknowns.
The signal transmitted by each satellite is a phase-modulated pseudorandom signal; the GPS receiver must acquire this signal. The satellite and the receiver both transmit the pseudo random signal at the same time, which is set by the general clock of the GPS system (the receiver generates a replica of this signal). The receiver then delays the start of this transmission until its signal is superposed on that coming from the satellite. This delay is determined by correlating the two signals. The value of this delay is the time taken by the signal to propagate from the satellite to the user. Because the time taken by the signal to travel this distance is of the order of 1/20 second, this type of measurement requires immense accuracy (better than 100 nanoseconds). However, as the clock of the GPS receiver is never totally synchronized to the clock of the satellites, to achieve the maximum correlation of the two signals the receiver has constantly to adjust its clock by a process of successive approximations. The acquisition of the signal therefore necessitates very considerable time scanning by the receiver.
In an Assisted-GPS (Assisted Global Positioning System) context, the position calculation process uses a mobile receiver capable of receiving and processing GNSS signals and of communicating with a cellular network, and an assistance data server responsible for broadcasting data for assisting the processing of GNSS signals in the mobile. An MS-Assisted (Mobile Station Assisted) mode of operation entails the server broadcasting data for assisting with the measurement of pseudodistances using GPS signals, the measured values being forwarded to the server, which calculates the position. The object of this mode of operation is:    to minimize the quantity of assistance data,    to lower the operating threshold of the receiver (in terms of signal to noise ratio), and    to reduce the computation power necessary for processing the GNSS signals.
The basic idea of the Assisted-GPS or Assisted-GNSS mode of operation is:    avoiding the receiver demodulating the ephemerides of the satellites contained in the signals coming from the satellites, which economizes on the time necessary for calculating the first point and the operating threshold, and    supplying the receiver with a prelocation, an idea of the time and the Doppler effect of the satellites, again to accelerate operation.
Nevertheless, in this mode of operation, typically referred to as the MS-Assisted mode, the mobile must send the pseudodistances to the server.
Because the position of the mobile is known a priori to an uncertainty equivalent to the size of the cell, it is possible to send only one item of spreading code phase information to minimize the traffic. Typically, in the context of the GPS SPS (GPS Standard Positioning Service), a spreading code having a period of 1 ms, the mobile sends the server the measured position of the beginning of a spreading code length in a data millisecond generally referenced to the beginning of a GPS system time millisecond. The server then deduces from the system time the position of the satellites at the time of transmission of the signal, thereby enabling the server to triangulate the user's position.
FIG. 1 shows the most immediate mode of operation, aiming to economize on processing by the mobile receiver. It is assumed that the mobile has access to the system time of the navigation system, in this example the GPS system time (reference 1). Various methods may be used to achieve this:    maintaining the time on an accurate local clock,    synchronizing to an external source synchronized to the GPS.
This is the case if the receiver is connected to a mobile network synchronized to the GPS.
To position itself, the receiver must measure the distance between itself and each visible satellite. The distance is measured by multiplying by c the difference between the transmission Time Of Week (TOW) and the reception TOW of the signal coming from the satellite. To this end, the satellite signal contains TOW information in a TOW message that has a period of a few seconds, typically 6 seconds in the GPS. The TOW information is relayed with a shorter repetition period by the very structure of the signal and to be more precise by the repetitions of the spreading codes (for conciseness, a spreading code is referred to simply as a code hereinafter). This information is independent of the TOW information contained in the navigation message and is ambiguous because it repeats with a certain period. This structure is represented at 1. In a GPS L1 situation, the spreading codes have a repetition period of 1 ms. They are represented at 2 for a signal received from a first satellite SV Observ. #1 (“Satellite Visible Observed #1”), at 3 for a signal received from a second satellite SV Observ. #2, and at 4 for a signal received from an nth satellite SV Observ. #nsat.
In the assistance situation represented in FIG. 1, i.e. in which the mobile has access to an external synchronization source (GPS clock 1), it is not necessary to solve the position to read the transmission TOW of the TOW signal in the message. It is in fact sufficient to measure the code phase of each satellite, i.e. to measure the time between the beginning of a receive code period expressed as a number of chips (6 to 8) and the millisecond transition on the GPS timescale (1).
Knowing:    a prelocation, and    the GPS TOW at which the measurement is effected, it is possible:    to calculate the approximate position of the satellites at the time of transmission of the signal, and    to deduce the approximate user-satellite distance, and consequently to resolve the 1 ms ambiguity as to the transmission TOWs.
On completion of the above process, the measured satellite-user distance is known unambiguously.
The quantity of information transmitted is reduced because there is only one time reference for the receiving time.
The above device is particularly attractive when the mobile has access to the GPS system time, typically in an IS95 mobile telephone network synchronized to the GPS. This is not the case in an asynchronous GSM telephone network. In a GSM network, the GPS system time may be obtained by reading the TOW field of the GPS message from the satellites, but this has a number of drawbacks:    it obliges the mobile receiver to demodulate a GPS message, which impacts on the time to calculate the point, and    demodulating the data requires a higher power of the received signal than simply detecting the beginning of a code.
Another approach would be to maintain the GPS time in the mobile by means of a local clock, but this presupposes having initial access to the information to reset the local clock. Also, the local clock of the receivers being of limited quality, an error of several tens of ms may affect the TOW, which leads to an error when the server calculates the position of the satellites and consequently to an error in the solution of the user's position.
To this end, the present invention proposes a method for minimizing the complexity of synchronization to the GPS.
A first embodiment of the present invention proposes to mark the code phase measurements relative to the TOW received in a particular GPS signal instead of relative to the GPS system TOW. This has the advantage of minimizing the number of TOWs to be demodulated. To this end, it suffices to identify the code transition TOW from only one received signal, in other words it suffices to demodulate only one received signal. The advantages of this are:    it minimizes the computation load for the mobile, and    the required power level for reception from the satellites is much lower than would be required to demodulate all the TOWs.